if a company was negligent but denies it, what's to stop them composing a backdated letter now and claiming they sent it to you at the time they should have? how can you prove they've just written it to save their skin?
Depends...if you really want to get into the nit and gritty then you can request them to prove it. The computer will have the dates the letter was written/adjusted.
Probably nothing at all, but as umms says, if it's that important their computers will know when it was written. Doesn't mean it was mailed that day, of course.